Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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; 1 49,334
A ~TU M~TER FOR MONIT~RING THE
HEATING VALUE OF FUEL GASES
BACKGROUND OF THE INVENTION
The total heatin~ value of a fuel gas is the sum
of the heating values of the individual combustible con-
stituents. Heating value tests are regularly performed by
utility gas companies to verify compliance of the fuel
with Public Service Commission regulations. Calorimetric
methods have been traditionally used to determine the
heating value of fuel gases. The calorimetric method
involves the burning of a definite volume of gas, absorb-
ing the heat liberated in a known weight of water andcalculating the heat content from the temperature rise of
the water. More recently, gas chromatographic analytical
methods ha~e replaced the calorimetric method for deter
mining heat values of fuel gases. The gas chromatograph
gives a ~lantitative analysis of the constituents of the
gaseous fuel. The heat value of the fuel is then calcu-
lated by using the gas analytical data and the known
heating values of the individual cons~ituents. ~hile the
gas cnromatographic method is more rapid and more conven-
ient than ~he calorimetric method, neither of the tradi-
tional methods are suitable for continuous, on-line mea-
surement of the heating value of the combustible con-
stituents ~f a fuel gas.
Inasmuch as it is anticipated that future fuel
gas supplies will be deri~ed from a variety of sourcas,
i.e., coal gasiflcation, ~itn the resultant varia~ions in
2 ~9,33~
heating values, there is a need for a technique whi.ch will
provide a continuous measurement of the heating value of
the fuel both for monitoring and process control purposes.
This technique has application in determining the ~'qual-
ity" of gas supplied to industrial and residential con-
sumers.
SUM~A~Y OF THE INVENTION
It has been determined experimentally that the
amount of oxygen consumed by the combustion of any gaseous
fuel or fuel mixture is an indication of the heating value
of the fuel. There is disclosed herein an implementation
of this relationship.
A sample of the fuel gas is mixed with a gas of
stable or known oxygen content, i.e., air, and the mixture
is supplied to a catalytic sensing electrode of a solid
electrolyte oxygen ion conductive electrochemical cell. A
reference of stable oxygen concentration, i.e., air, is
maintained at the reference electrode of the cell. The
catalytic combustion of the fuel/air gas mixture will
reduce the oxygen content and the resulting differential
oxygen concentration will produce a cell EMF signal which
is indicative of the oxygen consumed by the combustion
of the fuel gas. ThP oxygen consumption measurement is
indicative of the heating value of the fuel gas.
Alternatively the sample of the fuel gas can be
supplied directly to the catalytic electrode, and with the
cell operating as an oxygen pump, the oxygen transferred
from the reference electrode to the catalytic electrode to
react with the fuel gas will produce a cell current indic-
ative of the oxygen consumed by the combustion of the fuel
gas. This is a measurement of the heating value of the
fuel gas.
DESCRIPTION OF TIIE DRAWI~GS
l'he invention will become more readily apparent
from the following exemplary description in connection
with the accompanying drawings:
8~
3 4~,33~
Figure 1 is a graphical illustration of the
moles of o~ygGn required for combustion of fuel gases with
various heat values;
Figura 2 is a schematic illustration of an
embodiment of the disclosed technioue for determining the
heating value of fuel gases;
Figure 3 is a graphical illustration of the
equilibrium oxygen concentration after combustion of
monitored gas compositions consisting of 3 volume % fuel
gas/air mixtures, i.e., a mixture of 97% air and 3% fuel
gas; and
Figure 4 is a schematic illustration of an
alternative embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As shown in Table I below the heat of combustion
of most gaseous fuels divided by the number of moles of
oxygen (2) required for complete combustion of one mole
of fuel is nearly constant. It has been determined exper-
imentally that the amount of oxygen consumed by the com-
bustion of any gaseous fuel or fuel mixture is indicativeo the heating value of the ~uel. The oxygen consumption
calculations for some typical uel gases are illustrated
in Table II below. Further, a plot of this data showing
the correlation between oxygen consumption and heating
value is illustrated graphically in Figure 1. The heat of
combustion information presented in Table I as well as the
heating value information of Table III is discussed in the
"Handbook of Chemistry and Physics", 3rd edition, Chemi-
cal Rubber Publishing Co., 1961.
4 49, 334
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49, 334
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9 49,334
~or the purposes of discussion, consider a
mixture of 5% Ca4 in air at typical conditions of tempera~
ture and pressure One liter of this mixture contains
0.05 24.4 liter/mole = 0.00205 mole of CH4. Complete
combustion of this methane (C~4) reauires 2 x 0.00205 =
0.0041 mole 2' or 0.0041 mole x 24.4 liter/mole = 0.1000
liter oî 2 at room temperature and pressure. Thus, the
2 content of the original one liter o gas mixture would
be reduced from 20% to 10%. Since, as illustrated above,
the oxygen consumption during combustion is directly
related to the heat content of a fuel, the measurement of
the oxygen consumed, or that remaining, provides an indi-
cation of the heat content of the fuel.
An implementation of this novel techni~ue for
monitoring the heating value of fuel gases on khe basis of
oxygen consumed during combustion is typicall~ illustrated
in Figure 2. A sample of fuel gas from-a fuel gas supply
- line 10 is supplied to a fuel gas/air mixing apparatus 20
which mixes the sample of the fuel gas with a predeter~
mined amount of a gas of stable oxygen content, i.e., air,
to produce a fuel gas/air mixture which is supplied to a
commercially available oxygen/measuring detector 30. The
fuel gas/air mixture developed by the mixing apparatus ~0
is of a constant concentration with the concentration
being adjustable in terms of the air introduced into _he
mixing apparatus 20. While the fuel gas/air concert-ation
mixture can be adjusted to optimize the sensitivity of the
system for a particular heating value region, a 3 volume %
fuel gas/air mixture (97% air, 3% fuel gas) has been
selected for the pur~ose o~ discussion. Tne air content,
i.e., 97%, is typically chosen to assure sufficient oxygan
to completely combust the fuel at the detector 30 and
rasult in a residual oxygen in the mixture after combus-
tion.
49'334
The oxygen detector 30 is illustrated as con-
sisting of an electrochemical cell 32 having an oxygen ion
conductive solid electrolyte element 33 with a catalytic
sensing electrode 34 and an oxygen reference electrode 35
disposed on opposite surfaces thereof.
A furnace mPmber 36 maintains the electrochem-
ical cell 32 at an operating temperature of between 800
and 1000C to optimize the oxygen ion conductivity of the
solid electrolyte 33 and to assure a catalytic combustion
reaction between the oxygen and fuel constituents of the
gas mixture at the catalytic sensing electrode 34. The
electrodes 34 and 35 are typically platinum electrodes
with the platinum electrode 34 supporting catalytic com-
bustion of the oxygen and fuel constituents of the gas
mixture developed by the mixing apparatus 20. The result-
ing decrease in the oxygen concentration of the gas mix-
ture following the catalytic combustion reaction produces
a change in oxygen partial pressure across the electro-
chemical cell 32 and the resulting cell EMF is measured
electrically by a BTU meter 40 which is connected to the
electrodes 34 and 35 via electrical leads 42 and 43. The
electrical signal measured by the BTU meter 40 is mani-
fested as a measurement of the heating value of the fuel
gas flowing in the fuel gas supply line 10.
The operation of the solid electrolyte electro-
chemical cell 32 in both a pumping mode and in a potentio-
metric mode is describe~ in detail in U.S. Patent No. Re.
28,792, which is assigned to the assignee of the present
invention. The use of a solid electrolyte electrochemical
cell of the type described in U.S. Patent No. Re. 28,792
is illustrated in detail in U.S. Patent ~os. 3,791,936;
4,134,818 and 4,190,499, all of which are assigned to the
assignee of the present invention.
The amount of oxygen consumed, or that remain-
ing, following the complete fuel combustion of 3 vo~ume %
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11 49,334
fuel gas/air mixtures, such as that produced by the gas
mixing apparatus 20 of Figure 2, for the fuel gases listed
in Table II is presented in Table III below. The rela-
tionship of the equilibrium oxygen concentration of 'he
gas as monitored by the detector 30 after combustion, to
the heating values of a variety of commercial fuel gases
is graphically illustrated in Figure 3. The oxygen con-
sumed is a linear function of the heating ~alue of the
fuel gas as shown in the curves.
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13 49,334
~hile the discussion has been directed to flow-
ing gaseous fuel, the technique is equally applicable to
other f lowing fuel media such as li~uids an~ solid fl~els
including powdered coal supplied to a power plant.
An alternative embodiment of the heating value
measurin~ techni~ue is illustrated in ~igure 4. The fuel
gas/air mixing apparatus 20 and air source 22 of Figure 2
are eliminated and the fuel gas sample is supplied direct-
ly to the catalytic sensing electrode 34 of the cell 30.
~n this embodlment, however, a voltage is applied across
the cell 30 from a voltage source 38 to establish the cell
30 in a pumping mode o operation. The pumping action
transfers o~ygen from the oxygen reference, i.e., air, at
the reference electrode 35 through the cell 30 to the
catalytic sensing electrode 3~ to combustibly react with
the fuel gas sample from the supDly line 10. The transfer
of oxygen through the cell 30 produces a cell current.
The current value corresponding to oxygen required to
- effect the complete combustion of the fuel content of the
fuel gas is measured by the BTU measuring circuit 50 as a
measurement of the heating v~lue of the fuel gas. A fuel
gas flow rate control apparatus 60 is employed to maintain
the flow of the fuel gas to the detector 30 at a stable
level and limit the flow to a level which will permit the
cell 30 to effect complete combustion of the fuel gas at
the catalytic sensing electrode 3~.
